1. Field of the Invention
This invention relates to a valve mechanism. More particularly, it relates to a globe-style valve mechanism wherein an automatic actuator, e.g., of the pneumatic, hydraulic, or electrical sort, may be employed.
2. Description of the Prior Art
Heretofore, it has been common in cage-guided globestyle valves having automatic actuators to provide a hollow valve body in which the valve cage is retained, the cage being installed in the body through an opening in the body wall. To close this opening in the wall, a valve bonnet is employed which may be bolted or screwed to the body. A valve plug is commonly reciprocally disposed within the valve cage, and attached to the plug is a valve stem which extends through a set of packing in the valve bonnet to the exterior of the valve, the packing effecting a sliding seal between the bonnet and the stem.
Rigidly attached to the outer surface of the valve bonnet is the actuator. This actuator is usually equipped with an actuator stem concentric with and extending toward the valve stem, and the actuator stem and valve stem are connected together by any suitable means such as an elongated nut which engages threads on the ends of these shafts.
Valves which are generally exemplary of this prior art are disclosed in U.S. Pat. No. 3,572,382 to Luthe.
While such valves have been quite suitable in many application, nevertheless they suffer from certain disadvantages. In order to remove the plug-and-cage trim for repair or replacement in such a prior art valve, it has been necessary to disconnect the actuator stem from the valve stem, disconnect the actuator from its source of pneumatic or hydraulic pressure, and remove the valve bonnet and actuator from the valve body. Following this operation, the valve plug and the cage may be removed from the body. Typically, upon reassembly of such prior art valves the spacing between the ends of the actuator stem and valve stem must be carefully adjusted to insure that a desired predetermined seating force is applied by the actuator spring to hold the valve plug in a valve-closed position against the cage seat in the absence of a control pressure applied to the actuator. Moreover, in prior art valves in which the cage is threadedly retained at one of its ends by the inner portion of the valve bonnet, so that upon removal of the valve bonnet from the valve the plug and cage are removed with the bonnet, extreme care must be exercised in handling the bonnet and cage assembly (usually with the actuator attached), to prevent damage to the exterior of the cage during storage or during removal of the valve cage from the bonnet. A further common disadvantage of these valves has been that the pneumatic or hydraulic actuator control pressure piping must ordinarily be disconnected in order to remove the valve bonnet and trim.